INSTRUMENT OPERATION
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53
Chapter 6
Limitations
Unlike most patch-clamp amplifiers, the Axopatch 200B current clamp mode includes
Series Resistance Compensation. However, similar to many other patch clamps, current
clamp in the Axopatch 200B is not as fast or as stable as current clamp in a conventional
microelectrode amplifier such as the Axoclamp or the Axoprobe. This difference in
current-clamp performance results from significant differences in the design of the
headstages.
In a conventional current-clamp amplifier, the headstage is designed as a voltage follower.
Current is injected through a resistor and the pipette voltage is continuously recorded.
Alternatively, the headstage of a patch-clamp amplifier is designed as a current follower;
the pipette voltage is controlled while the pipette current is measured. To simulate current
clamp, a feedback circuit in the main unit of a patch-clamp amplifier automatically adjusts
the pipette voltage to keep the pipette current at the desired value.
Like any feedback circuit, the stability is compromised if the open-loop gain is too high.
When the headstage is grounded through a pipette, the voltage gain of the headstage is
nearly equal the value of the feedback resistor divided by the value of the pipette
resistance. In the WHOLE CELL mode and with a pipette resistance of 1 MΩ, this voltage
gain is 500 (50 for β = 0.1). In order to guarantee stability with pipette resistances as low
as 1 MΩ, the current-clamp circuitry must be deliberately slowed down, compromising the
response time for high-resistance pipettes. For low-resistance pipettes the main problem is
stability. In the extreme case of a zero-resistance pipette (i.e., a directly grounded input),
the enormous voltage gain of the headstage guarantees instability. To compromise between
the two conflicting requirements of speed and stability, the Axopatch 200B has been
designed with a dual speed current clamp. For low-resistance pipettes (between 1 and
10 MΩ) the I-CLAMP NORMAL setting will guarantee that the loop will be stable. For
higher resistance pipettes (above 10 MΩ), I-CLAMP FAST setting will be stable and have
a response many times faster than the I-CLAMP NORMAL setting. (It has been observed
that in some cases the I-CLAMP FAST setting will still be stable with pipette resistances
down to 3 MΩ). Risetime values for different combinations of pipette resistance and cell
membrane capacitance are listed in the
SPECIFICATIONS
chapter for current clamp.
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